This invention relates generally to dosimetry for a radiosurgery system employing multiple beams of radiation focused onto a stereotactically localized target, and more particularly to a dosimetry technique which quickly provides useful data for planning patient treatment.
In 1951, Dr. Lars Leksell coined the term "radiosurgery", to describe the concept of focusing multiple beams of external radiation on a stereotactically localized intracranial target. After experimentation with standard X-ray treatment devices, proton beam, and linear accelerators, he and his collaborators developed a device which is called the GAMMA KNIFE (currently marketed by the Electra Corporation, Stockholm, Sweden). The device consists of a hemispheric array, currently containing 201 Cobalt-60 sources. The radiation from each of these sources is collimated and mechanically fixed, with great accuracy, on a focal point at the center of the hemisphere. When a patient has a suitable lesion for treatment (usually an intracranial arteriovenous malformation), it may be precisely localized with another device called a stereotactic frame. Using the stereotactic apparatus, the intracranial target is positioned at the focal point of the GAMMA KNIFE. Since each of the 201 radiation pathways is through a different area of the brain, the amount of radiation to normal brain tissue is minimal. At the focal point, however, a very sizable dose is delivered which can, in certain cases, lead to obliteration of the lesion. This radiosurgical treatment is, in some instances, a much safer treatment option than conventional surgical methods.
Several GAMMA KNIFE devices are currently being used worldwide for stereotactic radiosurgery and have been used to treat approximately 1500 patients. The results of treatment, as well as many technical issues, have been discussed in multiple publications. Several factors, however, have impeded the widespread usage of this device. First, the device costs about $2.2 Million Dollars, U.S. Second, the Nuclear Regulatory Commission has ruled that this device cannot be shipped loaded in the U.S.A. Consequently, loading must be done on site, necessitating the construction of a portable hot cell. Third, the half life of Cobalt-60 is 5.2 years, which requires reloading the machine, at great expense, every 5-10 years. Fourth, the dosimetry system currently marketed with the device is relatively crude, especially when utilized with more modern imaging modalities such as CT scan and MRI scan.
An alternative method for radiosurgery involves irradiation of intracranial targets with particle beams (i.e., proton or helium). In this instance, one does not rely solely on multiple cross-fired beams of radiation. A physical property of particle beams, called the "Bragg-peak effect", allows one to deliver the majority of the energy of a small number of beams (approximately 12) to a precisely predetermined depth. Multiple publications regarding particle irradiation of intracranial lesions (especially pituitary tumors and arteriovenous malformations) have appeared in the literature. The results have not generally been as good as those obtained with the GAMMA KNIFE. This may, however, be solely a consequence of patient selection criteria. Particle beam devices require the availability of a cyclotron. Only a few such high energy physics research facilities exist in the world.
A third current radiosurgical method uses a linear accelerator (LINAC) as the radiation source. As mentioned above, Leksell rejected the LINAC as mechanically inaccurate. More recently, groups from Europe have reported their methods for radiosurgery with LINAC devices. In the United States, researchers at the Peter Bent Brigham Hospital in Boston have developed a prototype LINAC system using highly sophisticated computer techniques to optimize dosimetry. Thus far, approximately 12 patients have been treated with good results. This LINAC system, however, suffers from certain mechanical inaccuracies which have limited its use. In addition, the computer dosimetry system employed is very time consuming, rendering the treatment program inefficient.
Currently, there is great interest in radiosurgery. Although the GAMMA KNIFE represents the "gold standard", its great expense and requirement for frequent replenishment of radiation sources have discouraged most potential users. The proton beam devices are never likely to be widely available because of the requirement for high-energy particle beam source (cyclotron). The linear accelerator offers an attractive alternative to such devices. However, a major disadvantage of known linear accelerator based systems is the need for time consuming (e.g., several hours) computer calculations for determining the radiation distributions.
Before subjecting a patient to stereotactic radiosurgery, the tumor or other target area within the patient must be localized. This may be accomplished by stereotactic angiography or by CT (computer tomography) localization. After the localization of the tumor or other target area, a CT localizer (or an NMR imaging system) should be used on the patient, even if the original localization was using stereotactic angiography. The data from the CT scan and the angiographic films, if any, should be transferred to a computer system used for calculating the dosage.
When applying radiation to a patient, it is important that the radiation be concentrated on the target area and minimized for the patient's healthy tissues. It is especially important that the radiation be minimized on certain critical structures. For example, if using radiation treatment on a patient's brain, it may be important that the radiation dosage applied to the patient's optic nerves is minimized.
Before a physician applies the radiation to the patient, the physician may decide on two or more arcs which will be used for applying the radiation to the patient. In particular, the physician decides upon the plane in which the radiation beam will be applied in an arc to the patient's target area. The localization data and the proposed treatment arcs are input into a dosimetric computer system. That computer system generates a value for the radiation at each point in a grid extending throughout the patient's skull (assuming that the radiation is for the treatment of a target area within the brain). It is this process that is very time consuming and may require over four hours of computer time. Specifically, the process usually generates the value of the radiation dose at over 250,000 points within the patient's skull. After the doctor has received the radiation distributions from the computer, the doctor may decide that one or more critical structures is receiving too much radiation. Alternately, the doctor may decide that the target area is not receiving sufficient radiation. At any rate, the doctor may be required to revise the arcs through which the radiation source will travel in order to apply radiation to the tumor. It would then be necessary to repeat the very time consuming process of recalculating the radiation distribution.
Some prior dosimetric computer systems have been designed in which the radiation distribution may be calculated and shown or supplied for a smaller volume than the complete volume of the patient's skull. These type of systems require that one repeatedly indicate the area or volume for which the radiation distribution is desired. Although this may give faster results than the process giving the complete radiation distribution, the results are somewhat incomplete unless the doctor repeatedly selects numerous areas or zones for which the radiation distribution is requested. Each radiation distribution that is generated shows only a portion of the plane of view illustrating the radiation distributed within the patient.
The time-consuming nature of prior dosimetric systems is at least partly due to the generally used technique for calculating where the beam goes into the patient's skull. Specifically, the patient's skull may be simulated by thousands (often hundreds of thousands) of tiles and the usual "tiling" technique uses a series of simultaneous equations in order to calculate where the beam of radiation enters the patient's skull.
A further reason for the time-consuming nature of prior dosimetric procedures is that the resolution must be sufficiently high to give adequate details of the radiation distribution. In other words, the points at which the radiation dosages are given must be sufficiently close together that the doctor will have enough information to make proper decisions. On the other hand, this requirement for high resolution causes one to use so many data points that the calculations will, on most computers, take a tremendous amount of time.
A further reason for the time-consuming nature of previous dosimetric techniques is that such techniques require radiation distribution calculations based upon relatively complex mathematical models. The models require that the entrance width of the beam be taken into account because the width of the beam is generally large compared to the curvature of the patient's skull. In other words, the center of the radiation beam might be perpendicular to the patient's skull, but the beam is sufficiently wide compared to the curvature of the patient's skull that the edge of the beam is entering the patient's skull at a significantly different angle than at the beam center. Since the portion of the beam entering at the edge has a significantly different angle than the center of the beam, prior systems have generally taken into account this edge effect. This increases the complexity of the calculations. A further reason for the complexity of calculating the radiation distribution is that prior techniques usually require calculation of the primary radiation and the scattered radiation. The primary radiation is radiation which reaches a point inside the target volume with few interactions with the overlying material, whereas the scattered radiation is the radiation distribution resulting from the interaction of the primary radiation with the overlying structures or materials away from the primary path. The scattered radiation does not proceed along the same directional path as the primary radiation or the beam.
A further disadvantage of prior dosimetric systems is that they lack flexibility in terms of providing requested data.